To improve process control for some semiconductor manufacturing processes, integrated metrology (IM) modules are attached to the process tool and used to measure and quickly provide feedback for real-time control of the process. A typical IM module is built with the same form factor as a loadport, allowing it to be attached to the process tool EFEM (equipment front end module) in place of a loadport. In this case, the wafer handling robot, which is a part of the EFEM, can easily load wafers into the IM module. Typically, there is an open port between the IM and the EFEM, allowing the robot to freely load wafers into the IM.
In order to fit into the form factor of a loadport, the IM needs to be fairly compact. While typical stand-alone metrology tools may have a fan filter unit (FFU) to filter out particles, condition the air temperature, and provide laminar downflow, there is generally not enough space in the IM to include this FFU, so the conditioned air is provided by the EFEM.
In some cases, the metrology device in the IM module may be a spectroscopic reflectometer (SR), but other types of metrology devices may be used. Generally, the spectroscopic reflectometer measures the reflectivity of the wafer across a range of wavelengths. This information can be used to derive, for example, a film thickness of a thin film on the surface of the wafer, or the critical dimension (CD) of a device on the wafer. Reflectivity measurements typically require a reference reflectivity to be measured for calibration. By way of example, the reference reflectivity may be measured on a bare silicon reference chip. The reference chip may be mounted inside the IM chamber, e.g., in close proximity to the wafer.
With the IM module directly connected to the process tool, wafers may be loaded into the IM module immediately after they leave the processing module. Measuring wafers immediately after leaving the processing module allows for rapid process feedback, but in many cases, remnants from the processing are still present on the wafer, which may affect the performance of the metrology device. For example, in the case of an etch process, the etch gases may be absorbed by the processed wafer, and slowly leak out (outgas). For example, in the case of polysilicon etch, bromic acid (hydrogen bromate, HBrO3) is sometimes used. When wafers are measured after the etch process, the etch gases may leak out into the environment of the IM module.
The introduction into the metrology environment of process remnants may affect the optical properties of the reference chip. The reference chip is used to calibrate the metrology device, and thus, changes to the optical properties to the reference chip may adversely affect the measurement results. For example, gases that are used to etch silicon during processing, may outgas from the wafer and etch the bare silicon reference chip, thereby changing the surface properties of the reference chip. Other gases used during processing may condense on the surface of the reference chip, which may also change the optical properties of the reference chip.